This proposal seeks to elucidates the structure and function of iron- containing proteins involved in oxygen activation. The experimental techniques employed are 57Fe Mossbauer spectroscopy and electron paramagnetic resonance. (1) Studies of the iron-oxo proteins methane monooxygenase, delta9 desaturase, toluene-4-monooxygenase, ribonucleotide reductase, and alkane hydroxylase will be continued with particular emphasis on characterization of the electronic structures of the FeII-FeII states. Moreover, reaction intermediates will be trapped and studied. (2) This research will be complemented by studies of mononuclear and dinuclear complexes. (3) Many aspects of the proposed research involve the FeIV exudation state, either in intermediates of enzymatic reaction cycles or as transients in reactions of synthetic compounds. This proposal presents a systematic effort to study the spectroscopy of FeIV in proteins as well as in specially prepared synthetic complexes. (4) Studies of isopenicillin N synthase will be continued with special emphasis on trapping putative FeIV intermediates. (5) Cytosolic aconitase containing an Fe4S4 cluster exhibits enzymatic activity. As an apo-protein, it functions as the iron-responsive binding protein in the biosynthesis of transferrin receptor and transferrin. There is analytical evidence for the presence of an extra iron in addition to those contained in the Fe4S4 cluster. It will be attempted to identify and characterize the extra iron by Mossbauer spectroscopy. (6) Experimental and theoretical studies into the nature of the covalent link between the siroheme and the Fe4S4 cluster of E. coli sulfite reductase will be performed by investigating suitably linked synthetic assemblies. (7) Studies into the nature of antisymmetric exchange in model complexes for cyanide-inhibited cytochrome oxidase will be continued. (8) Finally, theoretical studies into the nature of the electronic structure of FeIII-FeIV mixed-valence dimers will be continued, with emphasis on exploring the role of double-exchange in compounds involving low-spin sites.